A number of problems can arise when outages occur within an electrical power system. One kind of problem, called voltage collapse, is a cascading loss of stability in subsystems caused by an equipment outage or increased load, which can in turn overload additional power system components. Voltage collapse can also be caused by an equipment outage due to relaying actions, failure of equipment or failure of control. Voltage collapse generally leads to a blackout or brownout where customers are no longer provided with the power they desire. Another type of voltage problem, called loss of voltage stability, is the inability of the generation, transmission and distribution system to supply enough power to satisfy customer demand after a disturbance, increased load or a change in operating conditions without an uncontrollable and progressive decrease in voltage. The source of these instability problems is within and between components in the electrical power system that delivers electrical power to customers. A third problem, called local blackout, occurs when the power network is pushed to its physical limits. This phenomenon can be caused by numerous factors, some of which are 1) exhaustion or depletion of reactive supply in the voltage control or reactive supply devices serving the local region or 2) outages of equipment or 3) increased power flow into or within a local region.
In an attempt to prevent voltage instability, methods have been developed that are directed toward identifying and rectifying weaknesses in the electrical power system before those weaknesses result in voltage instability. One such method is disclosed in U.S. Pat. No. 5,594,659 ('659 patent) entitled Method for Performing a Voltage Stability Security Assessment for a Power Transmission System.
While the '659 patent does provide a method directed toward identifying weak subsystems that evidence coherent behavior for the electrical power system, some drawbacks exist. Specifically, the '659 methods are intended to solely assess proximity to voltage instability in a coherent subsystem of a high voltage portion i.e. transmission level of a power network. The proximity measures developed in '659 are not intended to assess the vulnerability to voltage collapse or local blackout; or more importantly, to determine when the network has experienced voltage collapse and local blackout. The '659 patent does not address the structure within the transmission, sub-transmission and distribution networks required to produce sequential exhaustion of reactive reserves, sequential instability, voltage collapse, or local blackout or inability to obtain a loadflow solution. The '659 patent does not address a “divide and conquer” method of selecting contingencies that cause sequential exhaustion of reactive reserves, cascading instability, voltage collapse, or local blackout or that have no loadflow solution. In a preferred embodiment of the present invention, the Voltage Collapse Diagnostic, Preventive, Corrective and Emergency Control, and Available Transfer Capability described in this patent capture the structure needed to produce sequential exhaustion of reserves, sequential instability, voltage collapse, or local blackout or produce conditions that a loadflow has no solution. The present invention was developed in light of these and other drawbacks associated with the '659 methodology.